I just got back from Alaska, where I drove a US-spec Cayenne Diesel all over the place for two days. More importantly, I drove my '12 X5 xDrive35d to the airport so it was a back-to-back driving experience!

In a nutshell:

BMW is quicker
BMW is faster
BMW is quieter on the road
BMW is less expensive
BMW is roomier
BMW has a better AWD system

Porsche is more agile
Porsche is safer
Porsche is more fuel efficient
Porsche has a longer range
Porsche doesn't sound like a diesel

While I've driven the Euro-spec Cayenne Diesel extensively in Germany, the cars are actually quite different (both engine and transmission tuning). I immediately noticed the difference.

I've got to write the damn story, so I don't have time for a detailed post (ask questions).